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What are the three hemodynamic phases of acute unilateral upper tract urinary obstruction?
In the first phase of unilateral upper tract obstruction (first 1 to 2 hours), intrarenal pressure rises due to obstruction.
In response, tubuloglomerular feedback mechanisms allow for afferent arteriolar vasodilation (mediated by prostaglandin E 2 and nitric oxide), which increases kidney blood flow to maintain glomerular filtration rate (GFR).
In the second phase (lasting 3 to 4 hours), intrarenal pressures remain elevated, but elevated renin released during phase one results in angiotensin II mediated preglomerular vasoconstriction and decreased kidney blood flow.
A third phase (beginning ~5 hours after obstruction) is marked by a further decline in kidney blood flow and GFR mediated by a number of vasoconstrictors (including angiotensin II, thromboxane A 2 , endothelin).
When the normal flow of urine through the urinary tract is obstructed, urine accumulates, creating back pressure within the kidney’s collecting system. Over time, this pressure can cause dilation of the urinary tract structures and impair the kidney’s filtration function, leading to obstructive nephropathy.
The pathogenesis involves several mechanisms, including local ischemia from distention and elevated intratubular pressure. In cases of partial obstruction, there is an upregulation of angiotensin and AT1 receptors, which increases ureteral peristalsis in an attempt to overcome the blockage.
Although enhanced peristalsis may help in partial obstruction, in cases of complete obstruction, it can worsen the condition by increasing distention and intraluminal pressure. Animal studies, particularly in rats, have demonstrated the involvement of the renin-angiotensin-aldosterone system (RAAS), with a notable rise in TGF-beta1 expression in hydronephrotic kidneys.
This mechanism may also apply to humans, though more research is needed. These changes can result in permanent kidney damage, but the timing and progression are difficult to predict, underscoring the need for early detection and management of the obstruction.
References
Rishor-Olney CR, Hinson MR. Obstructive Uropathy. [Updated 2023 Jul 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK558921/
